Put the cream, sugar, lemon rind and vanilla seeds in a saucepan. Bring to the boil over a low heat, then remove from the heat and leave to infuse for 5 minutes.
Meanwhile, bring the milk to the boil in a separate saucepan. Remove from the heat and carefully sprinkle with the gelatine, in as thin and even a layer as you can manage. Leave to stand for 2–3 minutes until the gelatine no longer looks dry, then stir to dissolve.
Strain the infused cream through a sieve into the milk and pour the mixture into 6 x 175 ml (6 fl oz) dariole moulds. Cover each with clingfilm and chill for at least 5 hours until set. It will keep for up to 3 days in the refrigerator.
Combine the raspberries, mint and grappa in a bowl. Cover and leave to marinate at room temperature for 30 minutes and up to 4 hours.
Unmould the panna cotta by carefully dipping the base and sides of the moulds in warm water for a few seconds. Invert on to individual serving plates and serve with the marinated raspberries.